This application claims priority to Japanese Patent Application No. 199633/2000, filed on Jun. 30, 2000 in Japan. The contents of the aforementioned application are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method of operating a phosphoric acid fuel cell. In particular, the present invention relates to a method of operating a phosphoric acid fuel cell, which makes it possible to improve the energy conversion ratio of the phosphoric acid fuel cell and thus which makes it possible to allow the phosphoric acid fuel cell to generate the power at a high output.
2. Description of the Related Art
The phosphoric acid fuel cell has a power-generating cell which is provided with an electrolyte-electrode assembly comprising an anode electrode, a cathode electrode, and an electrolyte layer interposed between the both electrodes. The electrolyte layer is generally constructed such that pores of a porous silicon carbide member is impregnated with concentrated phosphoric acid (liquid electrolyte). However, another type of the electrolyte layer is also known, in which a membrane of basic polymer such as polybenzimidazole is impregnated with phosphoric acid or sulfuric acid (see U.S. Pat. No. 5,525,436).
In the phosphoric acid fuel cell, a predetermined number of the power-generating cells are electrically connected in series with each other to provide a fuel cell stack which is accommodated in a container. When the phosphoric acid fuel cell is operated, at first, the hydrogen-containing gas is supplied to the anode electrode, and the oxygen-containing gas is supplied to the cathode electrode.
The hydrogen in the hydrogen-containing gas is ionized on the anode electrode in a manner as represented by the following reaction formula (A). As a result, the hydrogen ion and the electron are generated.
2H2xe2x86x924H++4exe2x80x83xe2x80x83(A) 
The hydrogen ion is moved toward the cathode electrode via the electrolyte layer. On the other hand, the electron is extracted by an external circuit which is electrically connected to the anode electrode and the cathode electrode. The electron is utilized as the DC electric energy to energize the external circuit, and then it arrives at the cathode electrode.
The hydrogen ion moved to the cathode electrode and the electron arrived at the cathode electrode via the external circuit cause the reaction as represented by the following reaction formula (B) together with the oxygen in the oxygen-containing gas supplied to the cathode electrode.
O2+4H++4exe2x86x922H2Oxe2x80x83xe2x80x83(B) 
The reaction according to the reaction formula (B) is slow as compared with the reaction formula (A). That is, the reaction represented by the reaction formula (B) constitutes the rate-determining step in the overall cell reaction of the phosphoric acid fuel cell.
As described in Japanese Laid-Open Patent Publication No. 5-3042, U.S. Pat. No. 4,978,591, and Japanese Laid-Open Patent Publication No. 9-320620, the phosphoric acid fuel cell is generally operated in a state in which the pressure at the anode electrode is retained to be slightly higher than the pressure at the cathode electrode, for the following reason. If the pressure at the anode electrode is lower than the pressure at the cathode electrode, H2O, which is generated by the cathode electrode in accordance with the reaction formula (B), is pressurized toward the cathode electrode. For this reason, H2O is hardly removed. Therefore, the phosphoric acid in the electrolyte layer leaks into H2O, and the performance of the phosphoric acid fuel cell is deteriorated.
However, when there is such a difference between the pressures at the both electrodes, the ratio of oxygen is low at the cathode electrode. The reaction according to the reaction formula (B) is slow. For this reason, the energy conversion ratio of the phosphoric acid fuel cell is low.
If the pressure of the anode electrode is extremely higher than the pressure of the cathode electrode, a part of hydrogen in the hydrogen-containing gas supplied to the anode electrode passes through the electrolyte layer without being ionized, and it arrives at the cathode electrode. That is, the reaction efficiency of the reaction formula (A) described above is lowered. Therefore, the energy conversion ratio of the phosphoric acid fuel cell is low.
Further, in this case, phosphoric acid in the electrolyte layer leaks from the electrolyte layer together with hydrogen, and it is finally discharged to the outside of the container of the phosphoric acid fuel cell. Furthermore, the electrolyte layer itself is damaged in some cases. If the electrolyte layer is damaged, the performance of the phosphoric acid fuel cell is lowered.
The present invention has been made in order to solve the problems as described above, an object of which is to provide a method of operating a phosphoric acid fuel cell, which makes it possible to remarkably improve the energy conversion ratio of the phosphoric acid fuel cell and which makes it possible to avoid the leakage of liquid electrolyte such as phosphoric acid from a matrix and the damage of an electrolyte layer.
In order to achieve the object described above, the present invention lies in a method of operating a phosphoric acid fuel cell comprising an anode electrode to which hydrogen-containing gas is supplied, a cathode electrode to which oxygen-containing gas is supplied, and an electrolyte which is interposed between the both electrodes, the electrolyte including a matrix composed of basic polymer impregnated with acidic liquid electrolyte; wherein the phosphoric acid fuel cell is operated in a state in which a pressure on the cathode electrode is higher than a pressure on the anode electrode. The matrix referred to herein indicates a carrier for carrying the liquid electrolyte.
The matrix and the liquid electrolyte are strongly attracted to one another in the electrolyte layer comprising the matrix of the basic polymer impregnated with the acidic liquid electrolyte. Therefore, the matrix is excellent in ability to retain the liquid electrolyte. That is, the liquid electrolyte hardly leaks from the matrix in the electrolyte layer.
Further, the matrix, which is composed of the basic polymer, has the high mechanical strength as compared with the silicon carbide porous member. Accordingly, it is possible to construct the electrolyte layer which is hardly damaged. Further, the matrix has the low gas permeability. Therefore, the diffusion of the hydrogen in the hydrogen-containing gas toward the cathode electrode is remarkably suppressed before being ionized. Accordingly, it is possible to improve the energy conversion ratio of the phosphoric acid fuel cell.
The reaction represented by the reaction formula (B) described above is facilitated by increasing the pressure on the cathode electrode as compared with the anode electrode. Accordingly, it is possible to further improve the energy conversion ratio of the phosphoric acid fuel cell. Therefore, the phosphoric acid fuel cell effects the power generation at a high output.
It is preferable that a ratio between an absolute pressure on the cathode electrode and an absolute pressure on the anode electrode is not more than 2, for reliably avoiding the damage of the electrolyte layer and the leakage of the liquid electrolyte from the matrix.
It is preferable that a material, which has a structural unit of monomer of secondary amine, is used as the basic polymer, since such a basic polymer is excellent in ability to retain the liquid electrolyte, and the gas permeability of such a basic polymer is low as well.
When polybenzimidazole is used as the basic polymer having the structural unit of monomer of secondary amine, then the matrix is especially excellent in ability to retain the liquid electrolyte, and the gas permeability is remarkably low. Therefore, the use of polybenzimidazole is more preferable.
Preferred examples of the acidic liquid electrolyte are exemplified by any one of phosphoric acid, sulfuric acid, and methylsulfonic acid.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.